1. Technical Field of the Invention
The present invention relates in general to the mobile telecommunications field and, in particular, to a method and system for optimal routing of calls in a Base Station System (BSS).
2. Description of Related Art
FIG. 1 is a block diagram of an existing Global System for Mobile Communications (GSM) system model. Referring to FIG. 1, the GSM model (10) shown includes a Radio Access Network (RAN) known as a BSS (12). The BSS includes two types of logical nodes: a Base Transceiver Station (BTS) 14; and a Base Station Controller (BSC) 16. In order to support circuit-switched speech or data services, the BSC 16 inter-operates or interworks (“interworking” is a term of art) with a Mobile Switching Center (MSC) 18 via an open (non-proprietary) interface known as an A-interface. As such, an MSC (e.g., 18) can serve one or more BSCs.
Each BSC in a GSM network can control a plurality (typically hundreds) of radio cells. In other words, each BSC (e.g., 16) interworks with a plurality (hundreds) of BTSs via respective Abis interfaces. Each BTS (e.g., 14) is responsible for the transmission and reception of radio signals over an air interface, Um, in one cell. Consequently, the number of cells in a GSM BSS is equal to the number of BTSs in that BSS. As such, the BTSs are geographically distributed to provide adequate radio coverage of a BSC area, which forms part of a GSM Public Land Mobile Network (PLMN).
Additionally, the BTSs provide the capacity to carry a plurality of connections (calls) between Mobile Stations (MSs) (e.g., 22) and respective BSCs. In the GSM, each BTS is equipped with one or more Transceivers (TRXs). Each such TRX (not shown) is capable of handling eight timeslots of a Time Division Multiple Access (TDMA) frame. Furthermore, each such timeslot can be assigned different combinations of logical channels, such as, for example, Broadcast Control Channels (BCCHs) and Common Control Channels (CCCHs), Stand-alone Dedicated Control Channels (SDCCHs), and Traffic Channels (TCHs).
FIG. 2 is a block diagram of an Internet Protocol (IP)—based BSS 100, which has been developed by Ericsson. A more detailed description of such an IP-based BSS is disclosed in the above-described commonly-assigned, co-pending U.S. application for patent Ser. No. 09/494,606, the entire disclosure of which is incorporated herein by reference.
Referring to FIG. 2, the IP-based BSS 100 can include three types of nodes connected to an IP network 108. A first node connected to the IP network 108 is an RBS 102. In general, the RBS 102 functions similarly to existing RBSs used for implementing a GSM model. Moreover, the RBS 102 also provides IP support for the BSS 100. For example, the RBS 102 functions as an IP host and can include an IP router (not shown) . The IP router can be used to route payload User Datagram Protocol (UDP) datagrams to one or more Transmitter/Receivers (TRXs) and also for connecting a plurality of RBSs in various topologies.
A second node connected to the IP network 108 is a GateWay (GW) 104. The GW 104 can be used to terminate the A-interface. Also, the GW 104 can perform a conversion from one protocol (e.g., SS7 protocol) to another protocol (e.g., Transmission Control Protocol (TCP)/IP). The GW 104 can also include a Media GW (MGW) which functions similarly to existing Transcoder Controllers in an Ericsson implementation of the GSM model. The MGW (not shown) includes a pool of Transcoder/Rate Adaptor (TPA) devices (not shown), which, when allocated, are connected to the A-interface. However, the IP network (e.g., GSM) side of the TRAs in the MGW are connected to respective UDP ports. Preferably, the GW 104 is connected to the IP network 108 via a separate router (not shown).
A third node connected to the IP network 108 is a Radio Network Server (RNS) 106. The RNS 106 functions similarly to a BSC used for implementing a GSM model. A primary difference between the RNS 106 and a BSC is that the RNS does not switch payloads and does not include a Group Switch (GS). As such, the RNS 106 preferably carries signalling only, and includes a pool of processors (e.g., the number of processors determined by capacity requirements). The RNS 106 provides a robust, general purpose distributed processing environment, which can be based on a standard operating system such as, for example, SUN/Solaris™. The RNS 106 can serve one or more logical BSCs and is preferably connected to the IP network 108 via a separate router. As such, the payload can be routed directly between the GW 104 and RBS 102, without passing through the RNS' 106 processors. The A-interface signalling is routed between the RNS 106 and GW 104.
FIG. 3 is a block diagram of an implementation of a BSS, which can be used to illustrate the significant technical problems that need to be resolved. Referring to FIG. 3, in accordance with GSM Technical Specification (TS) 08.08, in a BSS (e.g., 200), all connections for the circuit-switched services are conveyed via the A-interface. As such, for example, if a speech call is being conducted between two parties in neighboring cells, the call is routed via the MSC 212. This routing occurs because the BSS 200 does not know that the two “half calls” (e.g., Signalling Connection-a 214 and Signalling Connection-b 216) belong to the same “full call” or conversation. This approach results in a so-called tromboning effect, which has significant disadvantages such as relatively high transmission costs, degraded speech quality, and longer delay. Consequently, with the increasing success and market penetration of mobile telephony, the number of mobile-to-mobile calls is expected to increase dramatically, and based on past experience, most of these calls will be local (i.e., within one BSS).
As illustrated by the BSS 200 shown in FIG. 3, in existing BSS implementations, semi-permanent circuit-switched connections are used between the BTSs 206, 208 and the BSC 210. The MSC 212 sends an Assignment Request Message to the BSS 200, which informs the BSS what circuit is conveying the “half call”. The Circuit Identity Code (CIC) Information Element (IE) in the Assignment Request Message provides the actual reference point information for the call. For example, the “half call” for Mobile Station-a (MS-a) 202 is associated with CIC-a 218, and the “half call” for MS-b 204 is associated with CIC-b 220. A signalling connection (e.g., Signalling Connection-a and -b 214, 216) is provided between MSC 212 and BSC 210 and MS-a 202 or MS-b 204 for each “half call”. In any event, MSC 212 is required to have complete control of the “full call” for a number of reasons, such as, for example, the MSC maintains the charging accounts, provides the dialling tone, and handles subscriber services (e.g., call transfer) . In any event, as described in detail below, the present invention successfully resolves the above-described problems, and also resolves other related problems.